(1) Field of the Invention
The present invention relates to a mono-component developing method for use in an image forming apparatus such as an electrostatic copier, laser beam printer etc., which uses electrophotography, more detailedly, relating a mono-component developer and mono-component developing method for use in a non-magnetic developing system in which a toner image is formed by contacting a developer support to a static latent image support.
(2) Description of the Prior Art
The image forming process using electrophotography implements image forming by the steps of charging, exposure, development, transfer and fixing. In the charging step, uniform charging is performed on the photoconductor surface containing photoconductive material. In the exposure step, an electrostatic latent image is formed on the photoconductor surface by exposure of the photoconductor. In the developing step, the static latent image on the photoconductor surface is visualized by the developer, forming a toner image. In the transfer step, the toner image on the photoconductor surface is transferred to a sheet. In the fixing step, the toner image is fixed to the sheet by heating, pressing and/or other means.
In the above different steps, as the developing system used in the developing step, the dual component developing system using a two-component developer composed of toner and carrier and the mono-component developing system using a single component developer made up of toner only, not containing carrier are generally known. Of these systems, the mono-component developing system is widely used from the viewpoints of maintenance, compactness, light-weight configuration, low-cost configuration etc.
The mono-component developing system is classified into the mono-component developing system using magnetic toner and the non-magnetic mono-component developing system using non-magnetic toner. Since the former uses toner added with magnetic material which is a black powder, a non-magnetic mono-component developing system is preferable for color image forming.
On the other hand, in the latter system, i.e., the non-magnetic mono-component developing system, the toner is liable to be degraded due to stress, hence the image quality is lowered due to long usage. Further, there is a problem that the degraded toner causes filming over the developing roller (developer support) and photoconductor (electrostatic latent image support). Here, ‘filming’ means fusion of toner or part of the toner composition to the surface of the developing roller or the photoconductor.
In effect, in the non-magnetic mono-component system, a doctor blade (film thickness control member) for regulating the amount of toner supported on and conveyed by the developing roller surface is provided along the longitudinal axis of the developing roller so as to oppose and abut the developing roller. The toner supported on the developing roller is adapted to pass through the nip between the developing roller and the doctor blade so as to form a uniform thin layer of toner (referred to as a toner layer) while the toner is triboelectrically charged. Accordingly, the doctor blade is abutted against the developing roller with a pressure which can triboelectrically charge the toner.
When the toner passes through the nip between the developing roller and the doctor blade, a strong stress acts on the toner, the toner being degraded. In the stressed toner, external additives (added agents to the base component in order to enhance the fluidity of the toner) may sink into the base toner (coloring powder) constituting the toner, or conversely the additives may come off from the base toner (coloring powder). As a result, the fluidity of the toner lowers or the cohesive force of the toner increases, causing the toner to fuse onto the developing roller and photoconductor, i.e., filming. Toner fusion to the doctor blade may also take place.
As the prior art for solving this problem, Japanese Patent No.2754539, Japanese Patent No.2759532 and Japanese Patent No.3127323 have proposed methods of adding an external additive of relative large-sized particles to the base toner. These patent publications disclose that the problems of the charge performance, fluidity or agglomeration etc., can be prevented.
It is true that each method of the aforementioned publications is effective in preventing lowering of the fluidity and filming by using an external additive of relative large-sized particles, but the external additive particles separated from the base toner build up on the developing roller surface, supply roller surface etc., causing gradual degradation of image quality from long term use.
Further, in recent years, there are demands for energy saving, color imaging, and oil-less fixing configurations in color imaging machines. In order to meet the needs, binder resins having relatively low softening temperatures and low melting point waxes are used for toner materials. While on one hand the particles of low melting point toner composed of low melting point resin and wax may present good performance in low-temperature fixing, the hardness of the particle surface of the toner is soft under usual temperature (under room temperature).
When such a low melting point toner is used as a non-magnetic mono-component developer, external additives are liable to sink into the toner particle surface, hence it is necessary to use large amounts of external additives of relative large-sized particles in order to prevent lowering of fluidity. As a result, the low-temperature fixing performance, which is the expected performance of a low-melting point toner, degrades. Therefore, it has been difficult to provide a toner which presents low-temperature fixing performance, prevention against degradation of fluidity due to embedment of external additive particles and prevention against instability of toner charging and supply due to buildup of separated external additive particles.